Patent document 1 describes an example of a conversion mechanism that is known as a rotational rectilinear motion conversion mechanism.
The conversion mechanism is provided with an annular shaft having an internal opening extending in the axial direction, a sun shaft arranged in the annular shaft and a plurality of planetary shafts arranged around the sun shaft. Further, an internally threaded section of the annular shaft is engaged with an externally threaded section of the sun shaft and an externally threaded section of the planetary shaft. Further, an internal gear of the annular shaft and an external gear of each planetary shaft are engaged with each other. In a conversion mechanism having such a structure, when the annular shaft rotates, the force transmitted from the annular shaft generates planetary motion of the planetary shafts around the sun shaft and linearly moves the sun shaft. In other words, rotational motion of the annular shaft can be converted to linear motion of the sun shaft.
In this case, threaded portions in the conversion mechanism are engaged at a plurality of locations. This causes the problems described next when manufacturing the conversion mechanism. When just joining the constituting elements (for example, a sun shaft and a planetary shaft) that include the threaded sections, if the threaded sections are not properly engaged with each other, it becomes difficult to join the constituent elements through just one joining operation. Accordingly, since the joining operation must be frequently carried out while finding the position or rotational phase at which the threaded sections of the constituent elements become engaged with each other, the assembly efficiency becomes low. Especially, when a gear is formed on the planetary shaft of the conversion mechanism as in patent document 1, the constituent elements must be joined while also engaging the gears. This further decreases the assembly efficiency.
Further, in a conversion mechanism that includes two gears for each of the annular shaft and planetary shaft, in which the corresponding gears are engaged with each other, and at least one of the two gears of the annular shaft is formed independently from a main body of the annular shaft, such as in the conversion mechanism of patent document 1, the problems described next occur. Since at least one of the two gears is formed independently from the annular shaft main body, the annular shaft may be assembled in a state in which the rotational phase of one gear relative to the other gear is greatly displaced. Further, when the gear of the annular shaft and the gear of the planetary shaft are engaged with each other in a state that greatly differs from the designed engaged state, the sliding resistance between the annular shaft and the planetary shaft may increase. This may lower the conversion efficiency of work from the rotational motion to the rectilinear motion. Such a problem also occurs when two gears are provided for each of the sun shaft and planetary shaft, and at least one of the two gears of the sun shaft is formed independently from a main body of the sun shaft.    Patent Document 1: International Patent Publication WO2004/094870